po4a: dummy test case git-debrebase_5.en_US.po: Create

This is the result of following the instructions in po4a/README on how
to start a translation.  For economy I have committed only this one
new .po file, even though po4a generated loads and normally a
translator would therefore send a PR with one file per document.

This is part of testing that the machinery works.  It is not practical
to make this a proper test case, but I can at least leave records in
the git history of how to do things.

This change results in that one manpage actually being generated and
installed, although it is very similar to the untranslated one.

I have verified that unless the translated manpages line in
debian/git-debrebase.install is uncommented, the package build now
fails, which is reassuring, because it means that when we get a real
translation we can't forget to actually ship it.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>